The present invention relates to an image forming method, and more particularly, to a method of forming an image on a light-sensitive material by exposing it under light from a semiconductor light source such as a light-emitting diode.
Techniques of image exposure of light-sensitive materials by scanning with semiconductor light sources such as light-emitting diodes (LED) and semiconductor lasers are described in many prior art references such as Unexamined Published Japanese Patent Application No. 151933/1982 Proc. of SPIE, 390, 149-154 (1983).
A light-emitting diode is a device that converts electricity to optical radiation by causing a forward current to flow through the circuit across the p-n junction of a semiconductor. Depending on the specific composition, light-emitting diodes emit electromagnetic radiation in a broad spectrum of wavelengths ranging from infrared to visible spectral regions. Light-emitting diodes can be made from a variety of semiconductor compounds such as GaAs (emitting infrared light), GaAsP (red), GaAsP:N (red, yellow), GaAlAs (infrared, red), GaN (blue), SiC (blue) and GaP (red, green). Using such semiconductor light sources, light-sensitive materials may be exposed for image formation by the following procedures: electrical signals for pictures either taken by video cameras or transmitted from TV stations, or electrical signals for original scenes picked up by light-receiving devices such as photoelectric tubes and CCDs (the latter type of electrical signals may be stored in a memory for further video processing), the electrical signals are used to control the radiation from light-emitting diodes or semiconductor lasers, and in synchronism with such signals, either a light-sensitive material of interest or such light sources or both are moved for exposure by scanning.
Semiconductor light sources have such distinct advantages over conventional light sources as small size, long life and low price. However, the radiation characteristics of semiconductor light sources are subject to variations in response to variations in the temperature at which the device is used (e.g., variations in the temperature of the device due to an external heat source or seasonal variations in the ambient temperature) or the self-heating of the device as a result of current flowing through it.
The first characteristic that is subjected to such changes is the radiation wavelength. Generally, a shift to a longer wavelength occurs as a result of self-heating of the device or an increase in the external temperature. The shift in radiation wavelength may be accompanied by a change in the half-value width, but more often is characterized by a shift of peak spectral radiation. For example, each increase of 1.degree. C. in the temperature of a light-emitting diode (whether the cause is self-heating or an external factor) will shift the peak spectral radiation to a longer wavelength by an approximate amount of 0.4 nm in a GaAlAs infrared light-emitting diode, 0.17 nm in a GaAlAs red light-emitting diode, and 0.1 nm in a GaP green light-emitting diode.
The second characteristic that is influenced by temperature variations is the radiation intensity of the semiconductor light source. For example, as the temperature of a light-emitting diode increases by 1.degree. C., its radiation intensity (L) may drop by .DELTA. log L.congruent.0.002-0.003.
Therefore, if the temperature of a light-emitting diode increases by 7.degree. C., the overall change in the amount of exposure (E) to which a light-sensitive material of interest is subjected will reach an approximate value of .DELTA. log E.congruent.0.1-0.2, taking into consideration the shift in radiation wavelength and the drop in radiation intensity. This value is due only to the temperature increase caused by self-heating of the diode, and will in actual cases be much greater because temperature increases due to external factors must also be allowed for.
If semiconductor light sources undergo such changes in their characteristics, they are no longer capable of providing the proper exposure for a light-sensitive material of interest and will cause such undesired effects as a reduced image density, uneven image density, and in the case of a color photographic material, fluctuations in the color balance of the formed image.
A method has been proposed for eliminating these problems by using a heater with which the temperature of the semiconductor light source can be held constant. However, this method is costly because of increased energy consumption. In addition, no consideration is made of the characteristics of the light-sensitive material which is to be exposed under the semiconductor light source. It has therefore been desired to develop an exposure method that is free from the defects of the conventional technique and which makes compensation for all the temperature-related factors that must be considered.
The principle object, therefore, of the present invention is to provide an image forming method that minimizes possible variations in the density or color balance of the finally obtained image by ensuring constant exposure conditions in the face of not only temperature-dependent variations in the radiation intensity and spectral characteristics of a semiconductor light source, but also the spectral sensitivity characteristics of a light-sensitive material exposed under such light source.